This invention relates generally to a process for making filaments of metal compounds and more particularly to a process for making silver metal filaments. The United States Government has rights to this invention pursuant to Contract No. DE-AC05-84OR 21400 with Lockheed Martin Energy Systems, Inc. awarded by the U.S. Department of Energy.
Particles of metal compounds may be made in many forms, including but not limited to, filaments, spheres, flakes and platelets, depending on their desired use. Metallic filaments, also called whiskers when single crystals, are characterized by relatively high tenacity and extremely high aspect ratio, i.e. high ratio of length to diameter. Much research has been done in the area of improving the performance of ceramics in the areas of cutting tools and engine parts by incorporating whiskers into the ceramic matrix. Another use for metallic filaments is as substrates for the deposition of high temperature superconductors. Silver in particular is a preferred substrate material for cuprate superconductors because of its resistance to oxidation at elevated temperatures in an oxidizing atmosphere. Such conditions are required to form the superconducting phase. Silver is also a preferred substrate material because it is chemically inert to superconducting materials. In addition, silver metal filaments may provide the necessary orientation for some superconductors to allow high current passage. For example, bismuth-lead-calcium strontium copper oxide and thallium-barium-calcium copper oxide superconductors require C-axis texture with some in-plane orientation.
Several methods for making silver metal filaments are known. These methods are based on several techniques including: (1) chemical reduction of silver salts (e.g. AgCl, AgI) by hydrogen and Ag.sup.+ by Fe.sup.2+ in aqueous perchlorate solution, (2) electrolysis of aqueous solutions (e.g. AgNO.sub.3) and of molten salts (e.g. AgCl, AgBr), (3) condensation of supersaturated silver vapor, (4) thermal decomposition of silver compounds such as Ag.sub.2 S, and (5) electrochemical growth from substrates such as Ag.sub.2 S. However, these methods are limited for use in the above described applications since the filaments produced by these methods have sharp bends and kinks and their resulting morphology cannot be predicted. Furthermore, a small number of filaments, sufficient only for microscopic examination, is produced. Accordingly, a need in the art exists for a process for making silver metal particles which produces particles of preselected morphology and consistently high yields.